1. Field of the Invention
The present invention relates to a method of forming a so-called mother to produce a so-called master used to mold an optical disc.
2. Description of Related Art
An optical disc comprises a transparent substrate having formed thereon a pattern of very fine concavities/convexities including pits and grooves indicative of information signals, a reflective layer provided on the substrate and formed from a metal film such as aluminum film or the like, and a protective layer provided on the reflective layer to protect the reflective layer from moisture and oxygen in the atmosphere.
For producing such an optical disc, a manufacturing process is required in which the optical disc can be replicated instantly with a high fidelity using a high-precision stamper.
To meet the above requirement, a photoresist, namely, a photosensitive resin, is applied onto a glass substrate, the photoresist is exposed to a laser beam to form a latent image corresponding to an information signal, and the photoresist on the glass substrate is developed to form a pattern of pits and grooves in the photoresist, thereby producing a mother. The pit/groove pattern is transferred from the mother to the surface of a metal substrate by electroforming or any other suitable method. The metal substrate thus processed is used as a stamper.
More particularly, a glass substrate having a thickness of about a few millimeters, for example, is applied uniformly on the surface thereof with an ultraviolet ray-sensitive photoresist by a spinner to produce a photoresist layer of 0.1-0.2 .mu.m. Next, while the glass disc is being spun, the photoresist applied thereon is exposed to a spot of a laser beam such as Ar (ion) laser, Kr (ion) laser or the like of 350-460 nm in wavelength generated in the blue or near ultraviolet zone and which is turned on and off correspondingly to an information signal to be written onto a final optical disc, thereby forming a latent image on the glass substrate. The photoresist on the glass substrate is developed to complete the pit/groove pattern in the photoresist, thereby forming a mother. Then, the pits/grooves pattern is transferred from the mother to the surface of a metal substrate by nickel plating. The metal substrate thus produced is used as a stamper.
Optical disc substrates can be replicated by the stamper in a large quantity by injection molding of a thermoplastic resin such as polycarbonate.
The amount of information recordable in an optical disc depends on the density at which pits or grooves can be formed in the disc. That is to say, the amount of information recordable in an optical disc depends upon how fine the pit/groove pattern can be formed by a so-called cutting process in which a photoresist layer is exposed to a laser beam to have a latent image formed thereon.
A stamper used to produce a read-only digital video disc (DVD-ROM), for example, has pit rows including shortest pits of 0.4 .mu.m in length spirally formed thereon with a track pitch of 0.74 .mu.m. An optical disc of 12 cm in diameter produced using this stamper has a storage capacity of 4.7 GB on one side thereof.
A Kr ion laser of 413 nm in wavelength is used for cutting the digital video disc. The length P of a shortest pit which can be formed in the disc can be determined from the relation (1) below: EQU P=K(NA/.lambda.) (1)
where .lambda. is the wavelength of laser beam, NA is the numerical aperture, and K is the process factor (depends upon the properties of a photoresist used, and usually takes a value of 0.8-0.9).
Therefore, for a digital video disc, putting .lambda.=413 nm, NA=0.9 and K=0.9 in the relation (1) will provide a shortest pit length of 0.4 .mu.m.
Along with the great progresses of recent technology of information communications and image processing, the aforementioned optical disc has been required to have a capacity several times larger than ever. For example, a DVD of 12 cm in diameter is required to have a storage capacity of 15 GB on one side thereof. To meet these requirements, the shortest pit length should be further reduced to 0.22 .mu.m and track pitch be to 0.41 .mu.m.
As seen from the above-mentioned relation (1), a decreased shortest wavelength of laser beam and an increased numerical aperture (NA) of an objective lens used are required for forming pits with such a high density. However, the currently available objective lens NA of 0.9 is nearly the upper limit because of the lens precision which can be attained in the design and manufacture of the objective lens. For example, when an ultraviolet laser of 250 nm in wavelength is used, a shortest pit length of 0.23 .mu.m is derived by using a process factor K of 0.8 in the relation (1).
Therefore, by cutting a photoresist having similar sensitivity and resolution to the conventional ones with respect to a far ultraviolet laser, it is possible to produce an optical disc having a storage capacity of 15 GB.
However, the conventional photoresist generally used in manufacture of optical discs, for example, novolak type photoresist, is conditioned to have a molecular design optimized for use in an exposure equipment using g ray of 436 nm in wavelength in the conventional photolithography originally used for manufacture of semiconductor devices. The light absorption will abruptly increase with a wavelength of less than 300 nm.
Thus, when the novolak type photoresist is cut using a far ultraviolet laser, the contrast value (.gamma. value) upon which the resolution depends is deteriorated due to a considerable light absorption in the photoresist, so that pits thus formed will have each a poor edge definition or a sloped edge profile. Further, since the photoresist is less sensitive to the far ultraviolet laser for the same reason, the cutting efficiency will considerably decrease. So, it is very difficult to cut the conventional photoresist as it is with a far ultraviolet laser.